This invention relates to a power brake unit for a hydraulic braking system including an auxiliary piston operable by servo pressure and a master cylinder whose piston is adapted to be acted upon in the operating direction hydraulically by pressure fluid displaced by the auxiliary piston and, in the event of failure of servo pressure, the master cylinder is mechanically displaced in the operating direction, with the diameter of the auxiliary piston being greater than the diameter of the piston of the master cylinder. Such a power brake unit is known from German Published Patent Application DT-OS No. 24 60 529 published July 3, 1975.
In this known power brake unit, the metered servo pressure acts in the operating direction on a diaphragm (or booster piston) which is supported on the auxiliary piston through an intermediate member, whereby the auxiliary piston is displaced in the operating direction when the brake is applied. Disposed between the auxiliary piston and the master-cylinder piston is a pressure chamber which is adapted to be closed by a valve arrangement. With the valve arrangement closed, movement of the auxiliary piston causes the pressure fluid in the pressure chamber to be displaced, thereby shifting the piston of the master cylinder by an amount which, proportionate to the diameter ratio between auxiliary piston and master-cylinder piston, is greater than the travel of displacement of the auxiliary piston. The valve arrangement is moved in an opening direction by the servo pressure so that it provides for communication between the pressure chamber and a reservoir when the servo pressure has failed. In this case, the diaphragm and thus the auxiliary piston are operated only by the driver's pedal force. However, because pressure is not able to build up between the auxiliary piston and the master-cylinder piston due to the valve arrangement being open to the reservoir, both pistons remain in mutual abutment and are displaced jointly by the same amount.
It is a principal disadvantage in this known power brake unit that such a valve arrangement must be provided for opening the pressure chamber when no servo pressure is available, in order to achieve different pedal-travel transmission ratios. It is another disadvantage in this method that the servo pressure, in the first place, is converted into a mechanical force in the booster portion by acting upon the diaphragm which in turn acts on the auxiliary piston. The forces are thus transmitted through a plurality of components. This is a particular disadvantage should the servo pressure fail, when the operator's pedal force, which is anyway rather low, is transmitted first to the diaphragm and then to the auxiliary piston through intermediate members before it eventually reaches the piston of the master cylinder. Due to the power losses occurring at each force-transmitting member, the efficiency of the power brake unit is adversely affected. Further, the operational safety is impaired due to the plurality of components inserted in the force path. Considering the known power brake unit as a whole, it is, in addition to having functional disadvantages, complicated and thus also expensive.